Electric injection mechanism

ABSTRACT

The object of the present invention is to reduce an excessive gnawing force at the thread engagement region of a ball screw portion with the ball nut member tending to generate in the electric injection mechanism by installing a stress relaxation means of annular groove at the shaft portion of the ball screw shaft, and prevent an abnormal wear or break of the ball screw shaft. A ball nut member  7  that the driving plate  6  of the screw comprises is engaged by thread with a pair of right and left ball screw shafts  4  rotatably provided in parallel with the guide bar  3  over the front and rear plates  1, 2.  The shaft portion  42  between the front plate  1  of the ball screw shaft  4  and the ball screw portion  41  is partially formed into a diameter smaller that the root diameter of the ball screw shaft  4,  and a stress relaxation groove  12  of concave curved surface having a flat bottom is disposed at the shaft portion  42.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention concerns an electric injection mechanism for assuring the screw forward driving by an electric motor.

[0003] 2. Detailed Description of the Prior Art

[0004] The electric injection mechanism, having an electric motor such a servomotor or the like as a driving source, for forward driving a screw by converting rotation movement by the electric motor into liner movement by a ball screw shaft and a ball nut member, includes one adopting a single ball screw shaft and one adopting a plurality of ball screw shafts.

[0005] In any of these types, a driver for driving a screw forward is provided between a pair of front and rear support plates on the machine base, by inserting into the guide bar, putting slidably on the installation member or the like. As the ball screw shaft rotating by the electric motor engages with the ball nut member installed inside the screw driver, the axial lines of the ball screw shaft and the ball nut member are misaligned relatively due to the attachment error of the ball screw shaft or lack of rigidity of the mechanism, causing radial force or moment easily.

[0006] In an electric injection mechanism adopting a single ball screw, the ball screw shaft fixation point is only disposed at the rear end born by the rear plate, and there is a flexibility at the meshing region with the ball nut member, therefore, it is the that a radial force due to a relative misalignment of axial line, or a gnawing force by the generation of moment is hardly caused.

[0007] However, in reality, the gnawing force is caused by the relative misalignment of the axis lines of the ball screw shaft and the ball nut member due to the fact that the screw driver is supported by inserting into the guide bar and that the rotatable ball screw shaft of the rear support plates engages by thread with the ball nut member installed inside at the rear middle of the screw driver by rotating the ball screw-shaft through the motor, and the screw driver is driven forward together with the screw.

[0008] Further, in an electric injection mechanism adopting a plurality of single ball screw shafts and ball nut members, as a pair of ball screw shafts are disposed rotatably in parallel with the guide bar, and the ball nut members disposed through at the right and left of the screw driver engages by thread with the ball screw shafts, the front and rear ends of the ball screw shafts are in a state fixed to the front and rear plates. As the screw driver is in a state supported through insertion by both of guide bars and ball screw shafts, the flexibility to the radial force or moment is inferior to the case of the single. If the parallelism between the guide bars and the ball screws shafts deteriorates even slightly, an excessive gnawing force is caused in the engaged region, and a premature wear, peeling off or other phenomena is caused easily in the ball screw.

[0009] Besides, a bending generates in the ball screw shaft and the ball screw shaft becomes to be supported slantingly by a bearing, thereby a problem of premature deterioration of the bearing member occurs.

[0010] Still further, in an electric injection mechanism linking front and rear guide plates by guide bars, a deflection may generate in the front plate and the guide bar and deform the mechanism on the advance and backward movement during the operation, whereby the ball screw shaft bends in a quadratic curb shape between the front and rear fulcrum points, the load distribution on a ball and a ball screw groove in the ball nut member becomes uneven, and an inclination generated causes an excessive gnawing force in the ball screw shaft and bearing portion and in the engagement region with the ball nut member, and the ball screw shaft may wear or deteriorates abnormally, and further even break by the stress concentration at this moment.

SUMMARY OF THE INVENTION

[0011] The present invention, devised to resolve the aforementioned problems in the ball screw shaft of the driving means adopted by the electric injection mechanism, has an object to provide a novel electric injection mechanism allowing to reduce the generation of an excessive gnawing force, by installing a stress relaxation means by an annular groove in the ball screw shaft.

[0012] According to the above object, the present invention is the one, comprising a screw driver disposed movably between a pair of front and rear support plates on the machine base, a driving means comprising of a ball screw shaft and a ball nut member disposed over the screw driver and the support plate, and an electric motor of the driving means, wherein a shaft between a ball screw portion and a bearing portion of the ball screw shaft is partially formed into a diameter smaller than a root diameter of a ball screw groove, and a stress relaxation groove having a bottom face of flat concave curbed face is formed at the shaft.

[0013] In addition, the present invention is the one comprising a pair of front and rear support plates disposed on the machine base by linking by means of a plurality of guide bars, a pair of right and left ball screw shafts disposed rotatably in parallel with the guide bar over the support plates, a screw driver provided right and left inside with a pair of ball nut members meshing with the ball screw shaft, and inserted movably into the guide bar between the support plates, and an electric motor of a driving means comprising of the ball screw shaft and ball nut member, wherein a shaft between a ball screw portion and a bearing portion of the ball screw shaft is partially formed into a diameter smaller than the root diameter of a ball screw groove, and a stress relaxation groove having a bottom face of flat concave curbed face is formed at the shaft.

[0014] Further, the present invention is the one comprising a pair of support plates disposed in front and behind an installation member on a machine base, a pair of right and left ball screw shafts juxtaposed rotatably over the support plates, a screw driver provided right and left inside with a pair of ball nut members meshing with the ball screw shaft, and put slidably on the installation member between the support plates, and an electric motor of a driving means comprising of the ball screw shaft and ball nut member, wherein a shaft between a ball screw portion and a bearing portion of the ball screw shaft is partially formed into a diameter smaller than a root diameter of a ball screw groove, and a stress relaxation groove having a bottom face of flat concave curbed face is formed at the shaft.

[0015] In the aforementioned configuration, an annular stress relaxation groove installed around the shaft portion of the ball screw shaft reduces an excessive gnawing force due to the generation of radial force or moment, or an excessive gnawing force tends to generate due to the deflection during the moment of advance or regression operation of the injection mechanism, resolves an excessive load at the engagement region of a ball screw portion with the ball nut member, and thereby, prevents an abnormal wear or deterioration, and further break of the ball screw shaft, maintains a smooth advance and regression operation of the screw driver, and extends the service life of the ball screw portion and the ball nut member.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1. a plan showing a part of the electric injection mechanism of the present invention by the portion;

[0017]FIG. 2. portion view along the line A-A of the same;

[0018]FIG. 3. a side view of the electric injection mechanism of another embodiment of the present invention;

[0019]FIG. 4. a portion view along the line B-B of the same; and

[0020]FIG. 5. a longitudinal partial portion view of the essential parts of the ball screw shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] The embodiment shown in FIG. 1 and FIG. 2 is an electric injection mechanism having a screw driver supported by inserting in both of the ball screw shaft and the guide bar, and in the drawing, 1 and 2 indicate a pair of front and rear support plates mounted oppositely on a machine base 14 by linking four corners by the guide bars 3, 3.

[0022]4, 4 indicate a pair of right and left ball screw shafts, born rotatably in parallel with the guide bars 3, 3 at the both side middles of the front and rear plates 1, 2.

[0023]5, 5 are electric motors by servomotors linked to the rear end of the respective ball screw shaft 4, 4, and attached to the outside side face of the rear plate 2.

[0024]6 is a screw driver, having a par of ball nut members 7, 7 installed inside at the middle of right and left sides, these ball nut members 7, 7 are engaged by thread with the ball screw shafts 4, 4, four corner thereof are inserted movably forward and backward into the guide bars 3, 3, and the screw driver is installed movably between the front and rear plates with the ball nut member 7 moving by the rotation of the ball screw shaft 4.

[0025] Moreover, the rear portion 9 of a screw in an injection heating cylinder 8 attached to the front middle of the front plate 1 is linked to the middle portion of the screw driver 6. The screw rear portion 9, not shown in the drawing, is installed rotatably in the interior of the screw driver 6, and liked to the tip of a coupling 10 fitted to a ball spline shaft rotationally supported at the middle portion of the rear plate 2, allowing to weigh the material by the screw rotation, by the ball spline shaft through this coupling 10.

[0026] The ball screw shaft 4 comprises a ball screw portion 41 engaged by thread with the ball nut member 7 of the screw driver 6, a required length of shaft portion 42 born by a bearing member 11 by a ball bearing of the front plate 1, and a support shaft portion 43 of the rear end born rotatably by the rear plate 2, and an annular stress relaxation groove 12 is disposed in a region between a bearing portion to the front plate 1 of the shaft portion 42 and the ball screw portion 41.

[0027]FIG. 3 and FIG. 4 indicate an electric injection mechanism wherein the screw driver 6 is slidably put on the installation board on the machine base 14.

[0028] It should be appreciated that same parts as the embodiment shown in FIG. 1 and FIG. 2 are indicated by the same symbol.

[0029] In this embodiment, a pair of front and rear plates 1, 2 linked by tie bars 13, 13 are installed on the machine base by putting and fixing the lower both sides on the installation board 16 put on the top face of the foundation 15 of the machine base 14.

[0030] A pair of ball screw shafts 4, 4 are installed by rotatively bearing in parallel with the tie bars 13, 13, at the both sides middle portions of these front and rear plates 1, 2. An electric motor (not shown) for injection driving is connected to the rear end of the ball screw shafts 4, 4.

[0031] Besides, a screw driver 6 is installed between the front and rear plates 1, 2, by slidably putting on the top face both sides of the installation board 16, without inserting the tie bars 13, 13, and a pair of ball nut members 7, 7 moving with the screw driver 6 on the ball screw shaft, engaging by thread with the ball screw shaft 4 is installed inside the middle at the right and left both sides of the screw driver 6.

[0032] A slider 17 serving also as centering member of the screw driver 6 is installed between front and behind the lower both sides of the screw driver 6, and the installation board 16. The slider 17 is comprising of a pair of wedge-shape members overlapping alternately by forming a contact surface respectively as a slant surface, and both of them are linked relatively movably by an adjustment screw 18 on the side.

[0033] The ball screw shaft 4 comprises a ball screw portion 41 engaged by thread with the ball nut member 7 of the screw driver 6, a required length of shaft portion 42, though omitted in FIG. 3, born by a bearing member 11 by a ball bearing of the front plate 1 as shown in FIG. 1, and a support shaft portion 43 at the rear end born rotatably by the rear plate 2, wherein an annular stress relaxation groove 12 is disposed in a region between the front plate 1 of the shaft portion 42 and the ball screw portion 41.

[0034] In any of the aforementioned embodiments, the stress relaxation groove 12 of the ball screw shaft 4 is installed by forming the groove shaft diameter (M) smaller than the root diameter (d) of the ball screw groove 44, and the bottom face is flat and is formed to make a globally concave curbed surface, as shown in FIG. 5.

[0035] The longitudinal length (L) of the stress relaxation groove 12 is determined affording a room in the length direction of the shaft portion 42 in a range of:

[0036] L=(D-M)/12×6 to 10

[0037] where, D is the outer diameter of the ball screw portion 42 and M is the shaft diameter of the groove.

[0038] In addition, as for the dimensions of the curvature (R) of the bottom face both sides and the curvature (r) of the corner, the radius of curvature is set as large as possible provided that no stress concentration generates.

[0039] The shaft diameter (M) of the groove is defined under the condition that the stress σM of the concerned region is superior to the stress ad of the root diameter and inferior to the allowable stress 9 Kgf/mm2 of a clamp screw portion 31 of the guide bar 3.

[0040] In short, it means

[0041] σd<σM<9 Kgf/mm2

[0042] The annular stress relaxation groove 12 installed around the shaft of the ball screw shaft 4 prevents a relative misalignment or bending of the axial line of the ball screw shaft 4 and the ball nut member 7 from generating, resolves an excessive load at the thread engagement region of the ball screw portion 41 with the ball nut member 7, and also by other effects, prevents an abnormal wear or deterioration, break or others of the ball screw shaft 4, maintains a smooth advance and regression operation of the screw driver 6, and extends the service life of the ball screw portion 41 and the ball nut member 7.

[0043] Though any of the aforementioned embodiments is the one provided with a driving means by a plurality of ball screw shafts and ball nut members, It should be appreciated that the present invention is not limited to the twin screw type electric injection mechanism, because it can also be applied to those provided with a driving means by a single ball screw shaft and ball nut member. 

What is claimed is
 1. An electric injection mechanism, comprising: a screw driver disposed movably between a pair of front and rear support plates on a machine base, a driving means comprising of a ball screw shaft and a ball nut member disposed over the screw driver and the plate, and an electric motor of the driving means, wherein: a shaft between a ball screw portion and a bearing portion of said ball screw shaft is partially formed into a diameter smaller than a root diameter of a ball screw groove, and a stress relaxation groove having a bottom face of a flat concave curbed face at the shaft.
 2. An electric injection mechanism, comprising: a pair of front and rear support plates disposed on the machine base by linking by means of a plurality of guide bars, a pair of right and left ball screw shafts disposed rotatably in parallel with the guide bar over the front and rear plates, a screw driver provided right and left inside with a pair of ball nut members meshing with said ball screw shaft, and inserted movably into the guide bar between the front and rear plates, and an electric motor of a driving means comprising of said ball screw shaft and ball nut member, wherein: a shaft between a ball screw portion and a bearing portion of said ball screw shaft is partially formed into a diameter smaller than a root diameter of a ball screw groove, and a stress relaxation groove having a bottom face of a flat concave curbed face at the shaft.
 3. An electric injection mechanism, comprising: a pair of support plates disposed in front and behind an installation member on a machine base, a pair of right and left ball screw shafts juxtaposed rotatably over the front and rear plates, a screw driver provided right and left inside with a pair of ball nut members meshing with said ball screw shaft, and put slidably on the installation member between the front and rear plates, and an electric motor of a driving means comprising of said ball screw shaft and ball nut member, wherein: a shaft between a ball screw portion and a bearing portion of said ball screw shaft is partially formed into a diameter smaller than the root diameter of a ball screw groove, and a stress relaxation groove having a bottom face of a flat concave curbed face at the shaft. 